Machines such as, for example, excavators, loaders, dozers, motor graders, and other types of heavy equipment use multiple actuators to move work tools connected to the machines. Recently, conventional hydraulic actuators have been replaced with electric actuators that can operate at higher efficiencies. An exemplary electric actuator includes a nut, and a screw shaft that drives the nut to travel linearly as the shaft is rotated inside the nut by an electric motor.
Although electric linear actuators may be more efficient than hydraulic actuators, the screw/nut mechanism in the exemplary actuator may be problematic. In particular, available screw/nut mechanisms employ large diameter screws. The larger the screw is, the thinner the wall of the associated cylinder. This relationship can limit a load-carrying capacity of the actuator. In some applications, conventional electric actuators use external gear trains to gain mechanical advantage and increase the load-carrying capacity. External gear trains, however, result in extra components, space, and cost.
One electric linear actuator having an internal reduction gear is described in U.S. Pat. No. 8,011,675 (the '675 patent) that was issued to Koichi Suyama on Sep. 6, 2011. The '675 patent describes an expansion actuator employing an internal screw feed mechanism formed from a male thread member and a female thread member. The male and female thread members are screwed one into the other and are driven by a motor via two reduction gears. The internal screw feed mechanism is disposed to surround the motor and the reduction gear. A relative axial displacement between the male thread member and the female thread member can be outputted as expansion and contraction due to relative movement of associated tubular housings.
Although the linear electric actuator of the '675 patent may solve some problems associated with conventional electrical actuators, it may not be optimal. In particularly, since the screw feed mechanism of the '675 patent surrounds the motor and the reduction gear, an expansion and contraction length range along the axis may be limited. Further, the utilization of two reduction gears renders the electric liner actuator of the '675 patent more complex and costly.
The disclosed electric linear actuator is directed to overcoming one or more of the problems set forth above.